WASHINGTON — The Obama administration's health care website stumbled, falling out of service for nearly four hours on deadline day for new sign-ups.

Visitors to HealthCare.gov on Monday morning saw messages that the site was down for maintenance. At times the visitors were also directed to a virtual waiting room, a feature designed to ease the strain on the site during periods of heavy use.

Administration spokesman Aaron Albright said the website undergoes "regular nightly maintenance" during off-peak hours and that period was extended because of a "technical problem." He did not say what the problem was, but a statement from the Department of Health and Human Services called it "a software bug" unrelated to application volume.

Officials said the website wasn't hacked. The site, which was receiving 1.5 million visitors a day last week, received about 2 million a day over the weekend.